How to get shell to self-detect using zsh or bash

Just do echo $0 it says -zsh if it's zsh and -bash if it's bash

EDIT: Sometimes it returns -zsh and sometimes zsh and the same with bash, idk why.

If the shell is Zsh, the variable $ZSH_VERSION is defined. Likewise for Bash and $BASH_VERSION.

if [ -n "$ZSH_VERSION" ]; then
   # assume Zsh
elif [ -n "$BASH_VERSION" ]; then
   # assume Bash
else
   # assume something else
fi

However, these variables only tell you which shell is being used to run the above code. So you would have to source this fragment in the user's shell.

As an alternative, you could use the $SHELL environment variable (which should contain absolute path to the user's preferred shell) and guess the shell from the value of that variable:

case $SHELL in
*/zsh) 
   # assume Zsh
   ;;
*/bash)
   # assume Bash
   ;;
*)
   # assume something else
esac

Of course the above will fail when /bin/sh is a symlink to /bin/bash.

If you want to rely on $SHELL, it is safer to actually execute some code:

if [ -n "$($SHELL -c 'echo $ZSH_VERSION')" ]; then
   # assume Zsh
elif [ -n "$($SHELL -c 'echo $BASH_VERSION')" ]; then
   # assume Bash
else
   # assume something else
fi

This last suggestion can be run from a script regardless of which shell is used to run the script.